Zero power memory cell with improved data retention

ABSTRACT

A method for forming a three transistor zero power memory cell including a p-channel sense transistor, an n-channel write transistor, and an n-channel sense transistor including: implanting a p-type impurity into a p-type substrate in which a n-channel high voltage transistor will be formed; implanting an n-type impurity into an n-type well in a p-type substrate in which a p-channel high voltage transistor will be formed; forming a mask to allow implants to occur to p-channel devices; performing a series of n-type dopant implants into the substrate where the p-channel transistors will be formed; growing a high voltage gate oxide; forming a mask to allow implants to occur to n-channel devices, said mask blocking implants to said n-channel sense transistor; and performing a series of p-type implants into the substrate where the n-channel devices will be formed. In addition, a memory cell which may include a first NMOS transistor having a source, drain and gate, and a first PMOS transistor is disclosed. The memory cell includes a first and second NMOS transistors, and a PMOS transistor, wherein the first NMOS transistor and first PMOS transistor each include a three implant channel region, and wherein the second NMOS transistor further includes a two implant channel region.

TECHNICAL FIELD

The present invention is directed to improvements in memory cells using no power, and in particular, to improvements in data retention in such cells by lowering the threshold voltage of one or more transistors used in the cell.

BACKGROUND

Nonvolatile memory cells are used in a variety of applications. As with many semiconductor device technologies, non-volatile memory device designers strive to increase the performance of devices, while decreasing device dimensions and consequently increasing circuit density. Designers also strive to reduce power requirements of devices by reducing program and erase voltage requirements.

Generally, arrays of individual memory cells are formed on a single substrate and combined with sense and read circuitry, and connected by row-wise and column-wise conductive regions or metallic conductors to allow for array wide bulk program and erase as well as selected bit programming.

Ideally, cells are designed to be reliable in retaining the state of their programming (either having charged or discharged floating gates) with no power attached to the cell.

Over time, the EEPROM memory cell will be written and erased repeatedly as data is stored and removed from the memory cell. Since the EEPROM memory cell relies on charge exchange between the substrate and the floating-gate electrode, considerable stress is placed on the tunnel oxide underlying the floating-gate electrode. The charge-induced stress in the tunnel oxide can cause charge trapping sites to form within the tunnel oxide. The formation of these charge trapping sites is undesirable because, once formed, electrical current can leak through the tunnel oxide layer from the floating-gate electrode to the substrate. When charge leaks off the floating-gate electrode a data error occurs in the EEPROM memory cell.

One solution to the tunnel oxide leakage problem is to form thicker oxide layers within the EEPROM device. By providing more oxide, the formation of a small number of charged trapping sites can be tolerated without deleterious current leakage in the device. While fabricating the oxide layers to greater thicknesses reduce charge leakage problems, the thicker oxide layers have the undesirable side effect of increasing the overall size of the EEPROM memory cell.

A need therefore exists for a way to improve data retention in memory cells without increasing their size.

SUMMARY

The present invention, roughly described, pertains to a method for forming a three transistor zero power memory cell including a p-channel sense transistor, an n-channel write transistor, and an n-channel sense transistor. In one aspect, the method includes: implanting a p-type impurity into a p-type substrate in which a n-channel high voltage transistor will be formed; implanting an n-type impurity into an n-type well in a p-type substrate in which a p-channel high voltage transistor will be formed; forming a mask to allow implants to occur to p-channel devices; performing a series of n-type dopant implants into the substrate where the p-channel transistors will be formed; growing a high voltage gate oxide; forming a mask to allow implants to occur to n-channel devices, said mask blocking implants to said n-channel sense transistor; and performing a series of p-type implants into the substrate where the n-channel devices will be formed.

In a further aspect, the invention comprises a memory cell. The memory cell may include a first NMOS transistor having a source, drain and gate, and a first PMOS transistor. The first PMOS transistor has a source, drain and gate, and the gate of the PMOS transistor is coupled to a floating gate region and said gate of said first NMOS transistor. In addition, the drain of said PMOS transistor is coupled to the drain of said first NMOS transistor. The memory cell further includes a second NMOS transistor, having a source coupled to a tunnel capacitor, the output of the tunnel capacitor coupled to the floating gate region. In a further aspect, the first NMOS transistor and first PMOS transistor each include a three implant channel region, and wherein the second NMOS transistor further includes a two implant channel region.

These and other aspects of the present invention will appear more clearly from the following description in which the preferred embodiment of the invention has been set forth in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to the particular embodiments thereof. Other aspects of the invention will become apparent with reference to the specification and drawings in which:

FIG. 1 is a schematic diagram of a three-transistor memory cell.

FIG. 2 is a plan view of an exemplary three-transistor memory cell formed in accordance with the present invention.

FIGS. 3A-3C are exemplary cross-sections of the transistors shown in FIG. 2 along lines AA′, BB′, and CC′.

FIG. 4 is a flowchart depicting a portion of the manufacturing process of a prior art three-transistor memory cell.

FIGS. 5A-5D are cross-sections of the process in FIG. 4 shown relative to the three transistors depicted in FIG. 2 and in cross-section in FIGS. 3A-3C, respectively.

FIG. 6 is a flowchart depicting a portion of the manufacturing process of a three transistor memory cell formed in accordance with the present invention.

FIGS. 7A-7G are cross-sections of the process of the present invention shown in FIG. 6 shown relative to the three transistors depicted in FIG. 2 and in cross-section in FIGS. 3A-3C, respectively.

DETAILED DESCRIPTION

In the present invention, an improved zero power memory cell, a method for improving the cell, and a technique which may be used to improve the zero-power memory characteristics of other memory cells are disclosed. In one embodiment, the voltage crossing a sense NMSOFET oxide during the cell's programmed state is lowered, without affecting any of the cell's zero power characteristics. As a result, data retention characteristics are improved.

FIG. 1 illustrates the configuration of a conventional CMOS memory cell 100 having a PMOS sense transistor 102 and a sense NMOS transistor 104 with a common floating gate FG. Drains of transistors 102 and 104 are connected together to form the output of the CMOS cell 100. Capacitor 106 is connected to couple voltage to the common floating gate. Bias voltage is provided to the source of PMOS transistor 102 from a chip product term (PT) pin. This voltage is otherwise referred to herein as the V_(d) of the cell 100. Bias voltage is provided to the source of the NMOS transistor 104 through a product term ground (PTG) pin, also referred to herein as the V_(s) of the cell. Capacitor 106 supplies voltage from an array control gate (ACG) node. An NMOS sense transistor 110 supplies a word bit line control (WBL) voltage to tunnel capacitor 108, as controlled by a word line (WL) voltage supplied to its gate.

Typical voltages applied for program, erase and read of the CMOS memory cell 100 are listed in Table 1 and the table of FIG. 8 below. In this application, programming indicates electrons are removed from the common floating gate, while erase indicates that electrons are added to the common floating gate. Table 1, below, shows how the voltages in FIG. 8 are generally applied to accomplish program, erase and read

TABLE I WBL WL ACG Vd Vs Program Vpp Vpp+ 0 0 0 Erase 0 Vcc Vpp+ Vcc Vcc Read Vcc/2 Vcc Vcc/2 Vcc 0

The CMOS memory cell 100 is advantageous because it enables zero power operation. Zero power operation refers to the fact that a component does not continually draw power when the component is not changing states. For instance, with an appropriate voltage applied to the common floating gate FG, PMOS transistor 102 will conduct and NMOS transistor 104 will not conduct. Current will then be provided from V_(d) (V_(c)) through PMOS transistor 102 to the output until the output is charged up to V_(c). In this configuration, no current will be provided through NMOS transistor 104 to V_(s). Further, with another voltage applied to the common floating gate 206, NMOS transistor 104 will conduct while PMOS transistor 102 does not. The output will then discharge to V_(s). No additional current will be provided through PMOS transistor 102 from V_(c) to V_(s).

FIG. 2 shows a plan view of a layout for the cell 100 of FIG. 1. While one such layout is presented, it should be understood that the invention is not limited to the layout shown in FIG. 2 and numerous alternative layouts may be utilized without departing from the scope and content of the present invention. FIGS. 3A-3C show respective cross sectional views at AA′, BB′, CC′, in FIG. 2. The layout for the CMOS cell shown in FIGS. 3A-3C is formed in a p type substrate having a typical background doping concentration of a P-type impurity of 10¹⁵-10¹⁷ cm⁻³, and is hence referred to herein as a P-substrate. Source and drain regions for the transistors, described below, are formed by, for example, any number of well known implantation and diffusion steps. In additional alternative embodiments, the substrate may comprise alternatives to bulk silicon materials well known in the semiconductor industry including, but not limited to, germanium, germanium silicon, gallium arsenide, polysilicon, silicon-on-insulator, or the like.

In the present invention, adjustments are made to the operating threshold voltages of the transistors 102, 104 and 110. Hence, details of the cell other than those pertaining to the construction of these transistors have been omitted for clarity. One of average skill in the art would readily understand the construction of various portions of cell 100 as such construction is widely known in the state of the art as exhibited by U.S. Pat. Nos. 5,587,945 and 5,596,524, which are hereby incorporated by reference.

As shown in FIG. 3A, NMOS write transistor 110 is formed by a polysilicon (POLY) word line (WL) region 216 on the substrate with a portion of region 216 overlying n+ implant region 210 and another portion overlying an additional n+ implant region 218. A substrate threshold adjustment implant region 302 is shown, formed by pre-doping the substrate with a plurality of doping steps via techniques illustrated with respect to FIGS. 4 and 5A-5D, or in accordance with the method of the present invention disclosed with respect to FIGS. 6 and 7A-7G.

As shown in FIG. 3B, NMOS sense transistor 104 includes two n+ implant regions 220 and 222 in the p substrate. A gate oxide region 224 of approximately 90Åis placed on the substrate bridging regions 222 and 220. The common floating gate 206 overlies the gate oxide region 224. In one embodiment, spacers may be provided as well as lightly doped drain extension regions 222 a, 220 a adjacent to the n+ implant regions. Substrate channel adjustment region 304 underlies the gate oxide and is formed by pre-doping the substrate with a plurality of doping steps via techniques illustrated with respect to FIGS. 4 and 5A-5D, or in accordance with the method of the present invention disclosed with respect to FIGS. 6 and 7A-7G.

As shown in FIG. 3C, PMOS sense transistor 102 includes two p type regions 230 and 232 included in a n-type well 236 which is included in the p type substrate. A gate oxide region 238 of approximately 90 Å is placed on the substrate bridging the regions 230 and 232. The common floating gate FG overlies the gate oxide region 238. Substrate channel adjustment region 306 underlies the gate oxide and is formed by pre-doping the substrate with a plurality of doping steps via techniques illustrated with respect to FIGS. 4 and 5A-5D, or in accordance with the method of the present invention disclosed with respect to FIGS. 6 and 7A-7G. Spacers and LDD regions may also be provided.

FIGS. 4 and 5A-5D illustrate a method of forming the transistors 102, 104 and 110 in accordance with the prior art, with transistor 110 at the left, transistor 104 in the middle, and transistor 102 at the right of FIGS. 5A-D. As will be generally understood by one of average skill in the art, only those processing steps affecting the channel implants to the substrate are illustrated, and numerous other processing steps are required in order to fabricate a complete device. Such additional processing steps are well within the knowledge of one of average skill in the art and are thus omitted here in order to not unduly cloud the features of the present invention.

It will be further generally understood that FIGS. 5A-5D show multiple steps represented in FIG. 4, and hence each of FIGS. 5A-5D are not single representations of an instantaneous temporal moment, but rather illustrate differences between the method of the present invention shown in FIGS. 6 and 7 and the prior art.

In the prior art method shown in FIG. 4, processing up to the point in the process wherein the p-channel adjustment implants normally used in the prior art is illustrated at process box 402. Such processing may include pre-doping, cleaning, annealing and other manufacturing steps utilized to form isolation regions and defined source and drain active regions. In one prior art process, processing box 402 includes an initial N-type threshold voltage adjustment implant which may be made to the NMOS write transistor 110 before additional threshold voltage adjustment implants, described below, are performed. This implant is performed by forming a mask layer to expose the substrate at a region where the NMOS write transistor 110 is to be formed, and implanting Arsenic at an energy of about 55 KeV to form an impurity region having a concenration of about 3.05×10¹² atm/cm².

Next, a group of steps 410 affecting the p-channel device threshold characteristics is performed. With reference to FIG. 5A and FIG. 4, at step 412, a p-channel device implant mask is formed over the surface of the substrate. Mask layer 412 is deposited over the surface of the substrate then photolithographically patterned and etched in accordance with well known techniques in order to expose only the so-called p-channel devices (devices whose operation is the result of the formation of a p-type channel in an n-type well or substrate), preventing implants into all but those devices formed in the n-well regions 236 as shown in FIG. 5A.

Following completion of the mask layer, a first threshold adjustment implant 414 is performed. Such implant is performed by well known techniques using a phosphorous (P) impurity at an energy of 250 KeV to provide a region having a concentration of 4×10¹² atm/cm².

Next, a p-channel punch through implant is performed at step 416. As will be understood by those of average skill, the punch through implant prevents the punch through effect where the depletion layers around the drain and source regions merge into a single depletion region. In this implant, for example, an Arsenic (As) implant at an energy of about 200 KeV and an implant angle of about 7° is used to provide a concentration of about 4.0×10¹² atm/cm².

A final p-channel implant is a relatively shallow implant to further adjust the threshold voltage. At step 418 in FIG. 4, a channel adjustment implant is performed using phosphorous at an energy of about 60 KeV and an angle of about 7° to form an implant region having a concentration of 3.2×10¹² atm/cm².

Next, as shown in FIG. 4 at process box 434, additional processing (such as, for example, the well known techniques for a “RCA” (Radio Corporation of America) chemical clean before oxidation of the HV transistors) will occur on the substrate up to the point of gate oxidation. As shown in FIGS. 4 and 5B, a gate oxide layer having a thickness of about 90Å is then grown on the surface of substrate 200. Oxide layer 432 is generally formed by immersing the substrate in an oxygen-containing atmosphere and heating the substrate for a period of time sufficient to grow the oxide to the desired thickness.

Next, as shown in FIG. 4 and 5C, an n-channel device implant mask 436 is formed on the surface of the substrate 200. With reference to FIG. 5C and FIG. 4, at step 434, a n-channel device implant mask is formed over the surface of the substrate. Mask layer 436 is deposited over the surface of the substrate then photolithographically patterned and etched in accordance with well known techniques in order to expose only the n-channel devices, preventing implants into devices formed in the p-substrate as shown in FIG. 5B.

Next, a group of steps 420 affecting the n-channel device threshold characteristics by forming n-channel implants 302, 304 is performed. Following completion of the mask layer, a first threshold adjustment implant 422 is performed. Such implant is performed by well-known techniques using a boron (B+) impurity at an energy of 115 KeV to provide an implant having a concentration of 4.4×10¹² atm/cm².

Next, a n-channel punch through implant is performed at step 424. In this implant, for example, an boron (B+) implant at an energy of about 50 KeV and an implant angle of about 7° to provide a concentration of about 4.0×10¹² atm/cm² is used.

A final n-channel implant is a relatively shallow implant to further adjust the threshold voltage. At step 426 in FIG. 4, a channel adjustment implant is performed using BF₂ at an energy of about 40 KeV and an angle of about 7° to form an implant region having a concentration of 3.2×10¹² atm/cm².

In accordance with the invention, to improve characteristics of cell data retention at zero power, the threshold voltage of NMOSFET 104 is lowered during the cell's programmed state. In general, the voltage at the gate of the sense MOSFET 104 is approximately equal to αV_(cg)+V_(fgp), where V_(fgp) is the floating gate potential of the programmed cell. V_(cg) is a reference voltage set automatically by off-chip circuitry. The value of V_(cg) is related to V_(pp) and the threshold voltage V_(T) of the NMOSFET 104 and the PMOSFET 102. V_(cg) is set to guarantee an appropriate on and off state during both the programmed and erased states. α is a coupling coefficient determined by the ratio of the ACG capacitor and sense transistor capacitor. Here α is equal to ˜85%. In the exemplary cell of the present invention following sense transistor V_(t) adjustment, V_(cg) is approximately 1.0 volt and V_(fgp) is 1.3 volts.

In order to lower the programmed state voltage, in one embodiment, both NMOS threshold V_(tn) and PMOS threshold V_(tp) are reduced by some amount, referred to herein as delta V_(t), so that the characteristics of transistors 102 and 104 such as the source/drain current (I_(ds))and the leakage currents (I_(off))will not be affected. Consequently, V_(cg) can be lowered by delta Vt.

A method for constructing the cell of FIG. 1 with a lowered sense transistor voltage during programming is shown with respect to FIGS. 6 and 7A-7G, with transistor 110 at the left, transistor 104 in the middle, and transistor 102 at the right of FIGS. 7A-G.

As shown in FIG. 6, the process of the present invention begins subsequent to the processing of the substrate up to the point in the process when the p-channel device threshold adjustment implants would normally occur as shown by step 402 in FIG. 6. As with the process illustrated in FIG. 4, the NMOS processing 402 may include an N-type threshold adjustment implant step for the NMOS write transistor 110 prior to the additional threshold implants set forth below.

Next, a N-channel adjustment mask and implant will be provided as illustrated at steps 604 and 606 in FIG. 6 and FIG. 7A.

As shown in FIG. 7A, the n-channel implant mask 604 will prevent implantation into the PMOSFET sense transistor and NMOS write transistor. Mask layer 604 is deposited over the surface of the substrate is photolithographically patterned and etched in accordance with well known techniques in order to expose only the substrate area where the NMOS sense transistor is formed. Next, at step 606, and as illustrated in FIG. 7B, an implant of boron at an energy of 25 KeV and a tilt angle of about 7° is used to form an implant dose of about 3×10¹² atm/cm² in the substrate in the channel area of the NMOS sense transistor.

Following the n-channel implant 606, the n-channel mask is removed and a p-channel mask and implant steps occur. As shown at FIGS. 6 and 7C, first, a p-channel device mask is formed 608 and prevents implantation to all but the substrate area where the PMOS sense transistor is formed. Next, a p-channel implant of phosphorous at an energy of about 55 KeV and an angle of about 7° to form an implant region of about 8.0×10¹² atm/cm² is performed in the channel area of the PMOS sense transistor.

Next, as shown in FIG. 6 and FIGS. 7D-7F, processing of the device including the p-channel implant step group 410, device processing 430, and oxidation 432 occur as in the prior art method shown in FIG. 4.

At step 636, the n-channel device mask layer normally used for the n-channel device adjustment implants is reconfigured to prevent channel adjustment of the NMOSFET sense transistor 104 as illustrated in FIG. 7G. The implants at group 420 are thereafter performed as to other NMOS devices (such as device 110), but do not affect device 104.

Processing of the device is thereafter completed as in the prior art process.

The following table shows the resulting change in the voltage across the oxide of the sense transistor when the voltage is reduced by delta V_(t) of 0.5 volts:

TABLE 2 Normal V_(t) reduced 0.5V V_(OX) cross Sense Transistor 2.20 1.77

A similar delta Vt is seen at the PMOSFET. As a result, in certain key aspects of the cell, lower voltages and improved data retention will result. For example, following programming the typical threshold voltage of the NMOSFET 104 and PMOSFET 102 will have decreased by about 0.43 volts on average, and may range from a delta Vt of 0.63 volts (at minimum voltages) to 0.53 volts at higher program voltages.

The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, the invention has been described with respect to particular technologies such as NMOS and PMOST used for certain transistors in the cell. It should be recognized that complementary transistors may also be used.

In addition, it should be recognized that the principle of the invention of reducing transistor threshold voltage may be applied to alternative embodiments of cells, including two-transistor cells. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto. 

We claim:
 1. A method for forming an EEPROM memory cell having an NMOS write transistor, a PMOS transistor and an NMOS sense transistor, comprising: (a) masking a p-type substrate to prevent implantation of impurities into the p-type substrate at a location where a PMOS device will be formed; (b) implanting a p-type impurity into the p-type substrate in which the NMOS sense transistors will be formed; (c) removing said mask formed in (a); (d) masking the p-type substrate to prevent implantation of impurities into the substrate at locations where NMOS devices will be formed; (e) implanting an n-type impurity into an n-type well region in the p-type substrate in which the PMOS transistor will be formed; (f) removing said mask layer formed in (d); (g) masking the p-type substrate to prevent implantation of impurities into the substrate at locations where NMOS devices will be formed; (h) performing a plurality of n-type implants into the n-type well region; (i) growing a high voltage gate oxide on the p-type substrate; (j) masking n-channel sense transistor and p-channel devices; and (k) performing a series of p-type implants into the p-type substrate.
 2. The method of claim 1 wherein (b) comprises implanting boron at an energy of 25 KeV and an angle of 7° to form an implant region of 3×10¹² atm/cm².
 3. The method of claim 1 wherein (e) comprises implanting phosphorous at an energy of 55 KeV and an angle of 7° to form an implant region of 8×10¹² atm/cm².
 4. The method of claim 1 wherein (h) comprises: implanting phosphorous at an energy of 250 KeV to form a region having an impurity concentration of 4×10¹² atm/cm²; implanting arsenic at an energy of 200 KeV and an angle of 7° to form a region having an impurity concentration of 4×10¹² atm/cm²; and implanting phosphorous at an energy of 60 KeV and an angle of 7° to form a region having an impurity concentration of 3.2×10¹² atm/cm².
 5. The method of claim 1 wherein (k) comprises: implanting boron at an energy of 115 KeV to form a region having an impurity concentration of 4.4×10¹² atm/cm²; implanting boron at an energy of 50 KeV and an angle of 7° to form a region having an impurity concentration of 4×10¹² atm/cm²; and implanting BF₂ at an energy of 40 KeV and an angle of 7° to form a region having an impurity concentration of 3.2×10¹² atm/cm².
 6. A method for forming a three channel zero power memory cell including a p-channel transistor, an n-channel write transistor, and an n-channel sense transistor, comprising: (a) implanting a p-type impurity into a p-type substrate in which the n-channel sense transistor will be formed; (b) implanting an n-type impurity into an n-type well in the p-type substrate in which the p-channel transistor will be formed; (c) forming a mask to allow implants to occur to the p-channel transistor; (d) performing a series of n-type dopant implants into the p-type substrate where the p-channel transistor will be formed; (e) growing a high voltage gate oxide on the p-type substrate; (f) forming a mask to allow implants to occur to the n-channel write transistor, said mask blocking implants to said n-channel sense transistor; and (g) performing a series of p-type implants into the p-type substrate where the n-channel write transistor will be formed.
 7. The method of claim 6 wherein (a) comprises implanting boron at an energy of 25 KeV and an angle of 7° to form an implant region of 3×10¹² atm/cm².
 8. The method of claim 6 wherein (b) comprises implanting phosphorous at an energy of 55 KeV and an angle of 7° to form an implant region of 8×10¹² atm/cm².
 9. The method of claim 6 wherein (d) comprises: implanting phosphorous at an energy of 250 KeV to form a region having an impurity concentration of 4×10¹² atm/cm²; implanting arsenic at an energy of 200 KeV and an angle of 7° to form a region having an impurity concentration of 4×10¹² atm/cm²; and implanting phosphorous at an energy of 60 KeV and an angle of 7° to form a region having an impurity concentration of 3.2×10¹² atm/cm².
 10. The method of claim 6 wherein (g) comprises: implanting boron at an energy of 115 KeV to form a region having an impurity concentration of 4.4×10¹² atm/cm²; implanting boron at an energy of 50 KeV and an angle of 7° to form a region having an impurity concentration of 4×10¹² atm/cm²; and implanting BF₂ at an energy of 40 KeV and an angle of 7° to form a region having an impurity concentration of 3.2×10¹² atm/cm².
 11. The method of claim 6 wherein (e) comprises immersing the substrate in an oxygen containing atmosphere to provide an oxide layer having a thickness in the range of 80-100 Angstroms.
 12. In a method for forming a memory cell having an NMOS write transistor and PMOS and NMOS sense transistors formed in a substrate, an improvement comprising: masking the substrate at areas where the NMOS write transistor and PMOS sense transistor are formed; implanting a p-type impurity into the substrate at a channel area of the NMOS sense transistor; masking the substrate at areas where the NMOS write transistor and NMOS sense transistor are formed; implanting an n-type impurity into the substrate at a channel area of the PMOS sense transistor; masking the substrate at areas where the NMOS sense transistor and PMOS sense transistor are formed; and implanting a p-type impurity into the substrate at a channel area of the NMOS write transistor.
 13. The method of claim 12 wherein implanting a p-type impurity into the substrate at the channel area of the NMOS sense transistor comprises implanting boron at an energy of about 25 KeV and an angle of about 7° to form an implant region of 3×10¹² atm/cm².
 14. The method of claim 12 wherein implanting an n-type impurity into the substrate at the channel area of the PMOS sense transistor comprises implanting phosphorous at an energy of about 55 KeV and an angle of about 7° to form an implant region of 8×10¹² atm/cm².
 15. The method of claim 12 wherein implanting a p-type impurity into the substrate at the channel area of the NMOS sense transistor comprises performing a deep n-channel implant, an n-channel punch through implant, and a threshold voltage adjustment implant.
 16. The method of claim 12 wherein the implants into the channel area of the NMOS sense transistor and PMOS sense transistor are such as to reduce the threshold voltage of each sense transistor by a same amount.
 17. The method of claim 12 wherein the implants into the channel area of the NMOS sense transistor and PMOS sense transistor are such as to reduce the threshold voltage of each sense transistor by about 0.5 volts. 